Person with the key, it is said, to some enigma (7)
I believe the answer is:
citizen
'person' is the definition.
(citizen is a kind of person)
'key it is said to some enigma' is the wordplay.
I cannot really understand how this works, but
'key' could be 'e' (a musical key) and 'e' is located in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
'is' could be 'ii' (more than one I) and 'ii' is located in the leftover letters.
This explanation may well be incorrect...
'with the' acts as a link.
Can you help me to learn more?
(Other definitions for citizen that I've seen before include "One belonging to a specific town or country" , "Person with full rights in country by birth" , "Resident" , "Native or naturalized member of a state" , "Zinc tie (anag.)" .)